This invention relates generally to communications systems and methods, and more particularly to communications systems and methods for the transmission of analog and digital information which employ overall channel synthesis and optimized detection techniques.
There is an increasing demand for low cost, reliable, communications systems for the transmission and reception of various types of analog and digital information. The increasing use by business and industry of computers for recordkeeping and the need to communicate information between commonly-owned facilities, the proliferation of commercial time-shared data bases and data processing facilities which sell their services to multiple subscribers, the advent of electronic banking, and the need for monitoring remote processing operations, such as remote well logging in the oil industry, as well as a host of other communications requirements have all contributed to this demand. The types of information communicated, i.e., voice, video or data, the speeds at which the information is communicated, and the types of systems, i.e., digital or analog, used to communicate the information are quite varied.
Many systems operate over short distances, as in metropolitan areas, and may employ either dedicated private or leased lines or point-to-point radio. Dedicated lines offer the advantage of providing a transmission channel with well-defined transmission characteristics and which is free from extraneous interferences such as fading, multipath or co-channel interference. However, dedicated lines are expensive to install and maintain or lease and, when used for data transmission, must be conditioned to equalize group delay. Recent developments in microwave components for use in the higher frequency ranges of the microwave spectrum, e.g., K-band (12-24 GHz), where large signal bandwidths are available for high speed digital transmissions and where the use of small highly directive antennas provide significant system and cost benefits, have contributed to an increasing use of microwave radio to satisfy communications requirements. Communications systems employing microwave links, however, are not free of problems. The system must be designed to accommodate fading, multipath and other extraneous interferences. Also, provisions must be made to compensate for carrier frequency drift, and, depending upon the frequency at which it operates, microwave radio may be subject to bandwidth or power limitations which affect the information that may be transmitted by the system.
Communications systems, regardless of whether they are used for voice or data and regardless of whether they employ dedicated lines or radio, are increasingly operating with digital information. As is well-known, digital communications systems, in general, require specialized techniques and systems that are tailored to the specific characteristics of the information being transmitted, e.g., data rate, or to specific modulation parameters in order to afford acceptable levels of performance. In recent years, a great deal of effort has been expended in the field of information theory in attempts to develop digital transmission systems and techniques that approach optimum performance under particular conditions. For example, modulation techniques such as differential encoding schemes, which affect the manner in which information is modulated on to a carrier, and various coherent detection schemes involving correlation or matched filter processes have been developed in order to minimize the probability of error in the detection process. Other systems have been proposed which minimize the bandwidth required for transmitting digital information at a specified data rate and probability of error, as shown, for example, in U.S. Pat. No. 4,135,057 to Bayless, Sr. et al., which discloses transmitter pulse bandlimiting networks and receiver pulse shaping networks designed in accordance with matched filter theory for affording pulse outputs of a predetermined shape. Still other systems attempt to afford digital transmission rates through a channel of given capacity that are greater than the maximun rate predicted by Nyquist theory. For example, the so-called partial response signaling systems proposed by Kretzmer in U.S. Pat. No. 3,388,330 transmit data at twice the Nyquist rate over a channel that has a partial response that extends over more than one symbol interval, and compensate for the resulting intersymbol interference in the data detection process by sampling the data in a predetermined manner.
Although such systems and techniques have resulted in significant improvements in digital transmission, in general, digital communications systems tend to be complex, expensive and rather inflexible in that they must be tailored to specific data characteristics, such as bit rate, and modulation parameters. As a result, a system designed for a specific set of characteristics and parameters may not be readily adaptable to a different set of characteristics and parameters. Accordingly, such techniques do not necessarily afford low cost systems, nor do they necessarily result in systems that are optimum.
It is desirable to provide communication systems and methods which avoid the foregoing disadvantages, and it is to this end that the present invention is directed.